1. Field
This disclosure relates to nonvolatile memories for use in electronic systems.
2. Description of the Related Art
A flash memory is a type of nonvolatile computer memory, which means that electrical power is not required to maintain the stored data. Flash memory typically offers lower cost for a given storage capacity than other types of solid-state nonvolatile memory devices. Thus flash memory has become the dominant technology for memory cards for digital cameras, USB flash drives, and lower capacity portable media players.
Flash memory device can be electrically read and written. However, only blank memory can be written. Previously written memory cannot be overwritten, but can be electrically erased and then rewritten. One type of flash memory is termed “NAND” flash memory. NAND flash memory can only be written in units composed of a plurality of bytes, commonly called “pages”, and can only be erased in even larger units composed of a plurality of pages, commonly called “blocks”. Flash memory can also only be read in pages, but some flash memory devices allow read access to smaller units of data, such as individual bytes, after the entire page has been read internal to the flash memory device.
NAND flash memory device typically offer relatively fast read access. Writing and erasing operations may be much slower than read operations. The time to write a page is may be three to ten times as long as a page read operation, and the time to erase a block is typically 70 to 100 times as long as a page read operation. In a conventional flash memory system, a read operation cannot be performed while a write or erase operation is in progress. In many applications, such as digital cameras and media players, relatively slow erase and write times are easily acceptable. In other applications requiring frequent exchange of data to and from the memory, the average throughput of a NAND flash memory may be very low compared to other solid-state memory devices.
Throughout this description, elements appearing in block diagrams are assigned three-digit reference designators, where the most significant digit is the figure number and the two least significant digits are specific to the element. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously-described element having a reference designator with the same least significant digits.